North Korea has produced a miniaturised nuclear warhead that can fit inside its missiles, US analysts have claimed.

Intelligence officials said the development meant the secretive state had crossed the threshold to become a fully fledged nuclear power, according to the Washington Post.

In a confidential assessment, seen by the newspaper, analysts said: "The IC [intelligence community] assesses North Korea has produced nuclear weapons for ballistic missile delivery, to include delivery by ICBM-class missiles."

It is not known whether North Korea has tested a mini-designed missile though the country claimed last year to have done so.

(Image: Getty)

The newspaper also reported that US analysts claim North Korean leader Kim Jong-un is now in control of up to 60 nuclear weapons following a recent assessment of Pyongyang's atomic arsenal.

But some independent experts are said to estimate the number of bombs is much smaller.

Japan today warned of the growing threat caused by North Korea's weapons programme amid a series of missile and nuclear tests in defiance of UN sanctions.

North Korea's official Korean Central News Agency (KCNA) has released images of missile tests carried out launched last month (Image: AFP)

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Japan released its annual Defence White Paper after North Korea fired two intercontinental ballistic missiles (ICBMs) last month on lofted trajectories to land off Japan's west coast.

The Defence Ministry said: "It is conceivable that North Korea's nuclear weapons programme has already considerably advanced and it is possible that North Korea has already achieved the miniaturisation of nuclear weapons and has acquired nuclear warheads.

Recent analysis has estimated Kim Jong-un is now in charge of 60 nuclear weapons (Image: AFP)